Information communication efficiency over an optical fiber transmission system, such as the Synchronous Optical Network ("SONET"), may be increased by optical wavelength division multiplexing. Wavelength division multiplexing ("WDM") systems employ WDM signals consisting of a number of different wavelength optical signals, known as carrier signals or channels, to transmit information over optical fiber. Each carrier signal is modulated by one or more information signals. As a result, a significant number of information signals may be transmitted over a single optical fiber using WDM signals. Channels within a WDM signal may be distinguished by either their optical wavelength or optical frequency, and the terms wavelength and frequency will be interchangeably referred to for this purpose.
Optical switching, multiplexing and demultiplexing, as well as the ability to add or drop signals at a specific wavelength, are critical components of a WDM optical network. In particular, optical add/drop filters are employed to facilitate the addition and subtraction of specific channels to or from the WDM signal at different locations on the network. Prior art add/drop filters generally consist of a 1.times.N demultiplexer followed by a N.times.1 multiplexer, where N corresponds to the number of channels in the WDM signal. After the first element demultiplexes the wavelength components of the WDM signal, the output ports of the channels to be dropped are connected to a drop line fiber. The remaining channels are then remultiplexed and may include the addition of a new channel that has the same wavelength of the dropped component but with a different information signal.
A drawback of these prior art devices is that each channel has to be processed or routed independently without any consideration as to whether any of the channels have the same destination. As a consequence, unwanted signal degradation is introduced into each channel by the narrow band filtering required for channel-by-channel processing, even if the channel is simply passing through the add/drop filter. Moreover, the complexity and cost of the add/drop filter is increased since channel-by-channel processing requires that each channel must have the identical hardware even if it is unused. A further drawback of these prior art devices is that they are not easily expandable to accommodate more channels. These latter two disadvantages become increasingly more important as WDM networks use a greater number of channels to increase the capability and capacity of the optical communications networks.